In recent years many efforts have been made to reduce or eliminate certain pollutants which are emitted in the exhaust gas of vehicles utilizing various types of internal combustion engines. These pollutants include carbon monoxide, various hydrocarbons and nitrous oxides.
Unfortunately, modifications to spark ignition engines generally cause the oxides of nitrogen to increase when the carbon monoxide and hydrocarbons are reduced and vice-versa. At the present time, it appears that the hydrocarbons and carbon monoxide can be controlled by devices such as catalytic converters or by providing precombustion chambers in which a rich mixture is ignited by spark ignition, the flame from the precombustion chamber then igniting a lean mixture in the main combustion chamber.
One way to reduce nitrous oxide emissions of a spark ignition engine is to operate at an extremely lean fuel-to-air ratio. Such lean operation will both increase engine efficiency and reduce pollutants. However, lean mixtures are difficult to ignite and as the mixture is made leaner and leaner the number of misfires increases causing increased amounts of carbon monoxide and hycrocarbon in the exhaust.
Satisfactory ignition is even more of a problem with rotaty type engines and stratified charge engines. Dual spark plugs and dual ignition systems are employed in some Wankel (rotary) engines to obtain the necessary burning pattern. The fuel charge in a stratified engine is difficult to ignite satisfactorily with a single spark or multiple short duration sparks.
In order to provide lean operation of a spark ignition engine without requiring the great complexity and expense entailed by using precombustion chambers, ignition systems have been developed to provide hotter and higher quality arcs across the spark gaps of spark ignition engines. One of these ignition systems is the well-known capacitive-discharge type wherein a voltage pulse having a sharp wave front and a magnitude several times that of conventional ignition systems is applied to the spark gap. The type of spark provided by the C-D type ignition system is extremely short and, while it may fire fouled spark plugs and minimize high voltage losses through leakage in cables and semiconductive deposits on the spark gaps, it does not provide maximum ignition of the lean fuel air mixture.
Some other types of ignition systems which provide multiple, repetitive sparking across the spark gaps have been developed to provide a higher probability of ignition of a lean fuel air mixture. It has been found that neither the C-D ignition system nor the multiple sparking ignition system is as effective in firing a lean fuel-air mixture in a spark ignition engine as a spark of extended duration.